1. Field of the Invention
The present invention relates to an optical scanner comprising an optical deflector using an electro-optic effect.
2. Description of the Prior Art
Generally, an optical scanner for use in a laser printer or a laser processing machine comprises a laser light source such as a semiconductor laser and an optical deflector deflecting laser light from the light source.
Mechanically driven mirrors such as a polygon mirror are used for the optical deflector. However, such an optical deflector has drawbacks of difficulty in downsizing and slow operation speed. Although it is possible to downsize the optical deflector by using a mirror manufactured by MEMS (Micro Electro Mechanical System) technology, there is a concern about the strength and durability of such a mirror and it cannot be driven at a high speed over MHz.
There is another type of optical deflector using an acoustic optical effect and comprising no mechanical driver. It requires an ultrasonic generator having a great output performance in order to achieve a large deflection angle, and great power outputs in order to increase the operation speed. This definitely complicates the drive system.
Utilization of optical switching technique by an electro-optic effect is one way to increase the operation speed of the optical deflector by a simple voltage driving. For example, an optical deflector using the electro-optic effect is formed by patterning polarization-inverse domains in the shape of prisms or the like on electro-optical crystals, to change a refractive index by voltage supply and deflect a light beam. The change in refractive index depends on a change in property of electrons in the crystals so that it can occur at a very high speed in order of GHz. Further, by controlling a refractive index by voltage supply, a drive circuit can be relatively simply structured. However, with a small change in the refractive index, there remains a problem that it is difficult to increase the number of resolvable spots without increasing the size of the deflector.
The number of resolvable spots of the optical scanner largely depends on the property of the optical deflector and is generally determined from a ratio of beam deflection angle and beam divergence angle. That is, to increase the number, it is necessary to increase the beam deflection angle and decrease the beam divergence angle.
The beam deflection angle is proportional to the optical path length of an index changing portion of the deflector and to a change in the refractive index of materials. The beam divergence angle is inversely proportional to a beam size. Accordingly, there are three possible ways to increase the number of resolvable spots: (1) increasing the optical path length of an index changing portion; (2) increasing the change in the refractive index of materials; and (3) increasing the beam size of light.
Aiming for elongating the optical path length of the index changing portion, a device concept of cascaded index changing portions has been proposed (Applied Physics Letters, vol. 81, No. 17, p. 3140, for example). Such a device can be made of available optical crystals such as lithium tantalite, lithium niobate. The index change of these materials is about 0.001 at most, so that the device needs to have a sufficient length in a light traveling direction to achieve a deflection angle. With a sufficiently long optical path length, even a small index change can increase the deflection angle, resulting in increasing the number of resolvable spots.
Next, with reference to FIGS. 20A, 20B, one example of increasing the optical path length of index changing portions is described. FIG. 20A shows an optical deflector 1 comprising a plurality of index changing portions 2 in the shape of inverted triangles arranged in a row and a rectangular electrode 3 surrounding the index changing portions 2. A light beam is irradiated to the optical deflector 1 from the left side in the drawing, deflected by the index changing portions and emitted from the right side of the optical deflector. The deflection angle here is θ1.
FIG. 20B shows an example of using three cascaded optical deflectors 1. By cascading them, a light beam is first deflected by a leftmost one and then deflected by the other two so that a large deflection angle θ2 about twice as large as the deflection angle θ1 can be achieved.
Further, for increasing the change in the refractive index of materials, Japanese Patent No. 3144270 discloses an optical deflector made of ferroelectric materials having a large refractive index change.
Furthermore, there is one way for increasing a beam size as shown in FIGS. 21A, 21B, for example. In FIG. 21A the beam size is set to be narrow (w1) while in FIG. 21B it is set to be wide (w2>w1).
However, there are problems with the above-described prior art techniques. The optical deflector shown in FIG. 20B configured to increase the optical path length of the index changing portions has a problem that the size of electrode supplying a drive voltage is increased three times as large as the standard size. Electro-optical materials are expected to achieve an extremely high-speed index change, using action of electrons in the crystals. Power consumed by transmitting signals at high speed greatly depends on the electrostatic capacitance of the entire circuit and an applied voltage. The electrostatic capacitance is proportional to the size of the electrode so that the larger the size of the electrode, the larger the power consumption, limiting the operation speed.
Moreover, materials with a large index change mostly exhibit extremely large permittivity. Since the permittivity is proportional to the electrostatic capacitance of the entire circuit, a large permittivity leads to increasing power consumption, limiting the operation speed. Besides, such materials are of an extremely particular kind and cannot be produced at low cost.
Further, with regard to increasing the beam size of light, the larger the beam size, the larger the size of the index changing region through which light is propagated. Accordingly, the size of a portion of the electrode changing the index, specifically, the height thereof (D1 to D2), need be increased as shown in FIG. 21B, increasing power consumption due to an increase in the electrostatic capacitance and limiting the operation speed, as in the above. Furthermore, the device has to include a long optical path length and a special optical system for modulating a parallel light beam from a semiconductor laser into one with a sufficient beam size, which increases the size of the device. It is also necessary to assemble the device at high precision since an allowable error in incident position of light with a large beam size on the optical deflector is very small.